U.S. Pat. No. 4,575,562 is directed to a process for the preparation of dimethyl adipate, which comprises reacting 1,3-butadiene with carbon monoxide under reactive temperature and pressure conditions in the presence of methanol, a catalyst which comprises a compound of the platinum metal group in a high oxidation state and an organic oxidizing agent such as a quinone, a dehydrating agent and preferably in the presence of one or more ligands, in order to form dimethyl hex-3-ene dioate inter alia, followed by hydrogenation of the dimethyl hex-3-ene dioate under the formation of dimethyl adipate. This reaction should preferably be carried out in the presence of palladium compounds. Preferred examples of ligands to be employed are: triphenyl phosphine, tri(p-methoxyphenyl) phosphine, tri(p-fluorophenyl) phosphine, tributyl phosphine, triphenyl arsine, triethyl arsine, benzonitrile, acetonitrile, propionitrile, valeronitrile, succinonitrile, glutaronitrile, triphenyl phosphite, lithium chloride, sodium bromide, lithium iodide, potassium iodide and copper chloride. Furthermore, a small quantity of an acid selected from acetic acid, trifluoroacetic acid, sulfuric acid, hydrochloric acid, hydrobromic acid or hydroiodic acid could also be employed with this process.
From the butadiene conversion and the selectivity for dimethyl hex-3-ene dioate reported in the examples of this patent, however, it will be clear to a person skilled in the art that this process cannot be considered as a particularly selective preparation with a high yield (30%, calculated on butadiene) of these desired alkene dicarboxylic acid esters, which, as starting materials for further chemical synthesis such as the preparation of adipates are becoming increasingly important. Moreover, this process is performed at relatively high pressures (34-350 atm.) and temperatures (60.degree.-190.degree. C.), which makes additional demands on the apparatus to be used and thus leads to higher costs.
It is known from International Patent Specification WO 80/00250 to convert conjugated diolefins (for example 1,3-butadiene) into esters to alkene polycarboxylic acids (for example dimethyl hex-3-ene dioate) by the reaction with carbon monoxide and an alcohol (for example benzyl alcohol) in the presence of a palladium catalyst, a copper(II) salt and a base, whereupon this unsaturated diester can be hydrolyzed and hydrogenated or vice versa in order to prepare a corresponding linear di-acid (for example adipic acid). The copper(II) compound should be employed in a quantity which is sufficient to oxidize the palladium(O) formed by this process back to palladium(II), and the quantity of nucleophillic base should be at least one molar equivalent of the copper(II) salt. As a copper salt, copper chloride is preferably employed. Preferred bases are alkali and alkaline earth metal salts of carboxylic acids or carbonates such as sodium acetate, potassium acetate, sodium propionate, sodium butyrate, sodium carbonate or amines such as triethylamine or lutidine.
From the examples of the process described above, it will be clear to a person skilled in the art that neither can this process be considered as a particularly selective high-yielding process for the preparation of the desired alkene dicarboxylic acid ester, such as dimethyl hex-3-ene dioate. Furthermore, the aforesaid process is characterized by relatively long reaction times (144 hours in Example I). It is therefore an object of the present invention to provide a selective conversion of butadiene of homologues thereof to the desired alkene dicarboxylic acid diester, such as diethyl or dimethyl hex-3-ene dioate and/or dimethyl hex-2-ene dioate in order to supply the ever growing need for cheaper fine chemicals as starting compounds for further chemical synthesis.